Leave allowances are governed by German law. As a consequence, you have to pay leave scheme contributions to SOKA-BAU when posting commercial workers to constructions sites in Germany. Such contributions supersede any leave scheme contributions you may pay in your home country during the posting period. Commercial workers are therefore not entitled to two leave allowances.
Exceptions to this rule only apply if you pay contributions to a comparable leave scheme in your home country. There are mutual recognition agreements to this effect. At present, this applies to leave schemes in Belgium, Denmark, France, Italy and Austria.